Chaz Andrews, Technical Manager at Doepke, discusses RCD circuit protection associated with Mode 4 charging equipment.
If you’re involved in regular installation of DC EV chargers, you’ll already be familiar with the significant differences between individual manufacturers’ performance characteristics and the impact on the supply-side equipment e.g. transient inrush current, harmonic distortion, leakage current, and associated RCD selection.
Mode 4 guidance
It is not possible to give standardised recommendations for RCDs associated with Mode 4 chargers. But why?
Whilst there has been agreement for Mode 3 charging standards in Europe, with the publication of ENIEC 61851-1 2017, and the associated changes in section 722 BS 7671 2018, that is not the case with Mode 4 charging standards which are still coalescing. DC charger design is far more complex due to the safety issues associated with high DC voltage/current. This is reflected in the time taken to agree on a revision of BS ENIEC 61851-23 2014 – the current designated standard for DC charging, quoted in BS 7671.
For example, this early standard gives general design requirements but is light on standardised test methods for checking conformity. This results in a wide variation in basic standardised performance characteristics between individual manufactures claiming compliance with a designated standard.
Which standard?
The current edition of BS 7671 (Oct 24) Clause 722.531.3.101 relating to RCD selection Note 2 states that: supplies using DC vehicle connectors to the BS EN 62196 series are under consideration. Electrical safety design characteristics (the guts of the charger) may depend on the DC interface charging technology adopted by the vehicle manufacturer e.g. CHAdeMO (Japanese), GB/T (China), CCS 1 & 2 (North America & Europe), and Tesla (proprietary design NACS – based on North American standards).
Later versions of Tesla DC chargers were supplied with CCS 2 interface (compatibility with European EVs), but still NA electrical design standards.
Use the basic principles of BS 6761 (clause 133.1, 134.1.1, and 531.3.3) to select appropriate RCDs, based on the individual chargepoint manufacturer’s characteristics and installation recommendations e.g. note minimum RCD characteristic requirements at the quotation stage.*
* Clause 642 (Inspection): Refer to as an aid-memoire during installation planning/equipment verification.
Mode 4 chargers – general points
RCDs installed on the AC supply feeding the charger does not provide protection on the DC side of the charger. Electric shock and fault protection on the DC side is the responsibility of the chargepoint manufacturer: Conformity with the essential safety requirements is indicated by CE/UKCA Marking – see Clause 642. Refer to the chargepoint manufacturer’s installation instructions.
Inrush/transient currents
Depending on design and technology employed, equipment containing high power rectifiers/inverters can produce significant transients during operation. Check with the equipment manufacturer – RCDs may require transient resistant features, to prevent unwanted tripping and unnecessary equipment downtime.
AC leakage current
This is the current that flows to earth during normal operation. Leakage current values are specific to the manufacturer’s design and will vary as a function of the individual chargepoint harmonics, produced during various stages of charging and the supply quality (existing harmonic content).
The existing standard BS ENIEC 61851-23 sets minimum protective conductor requirements for Class 1 equipment, where touch currents exceed 3.5 mA. Follow the manufacturer’s recommendations if they exceed the requirements of BS 7671 543.1 – 543.7.
Unexplained RCD tripping may be the result of insufficient safety margin between the operational leakage current and the RCD sensitivity; note recommendations in 531.3.2 (ii).
RCD Type
Until recommendations are included in BS 7671-722, unless otherwise stated by the chargepoint manufacturer, only Type B RCDs (RCCB, RCBO, CBR) should be used upstream of Mode 4 chargepoints – see 531.3.3 (iv).
Conclusion
With any innovative technology the “state of the art” is an important concept to consider about the existing requirements of BS 7671 and installation advice provided by the equipment manufacturer. Ask before acting and keep in mind the requirements of clause 133.1, 134.1.1 and 531.3.3.
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